Optical recording medium put to practical use are roughly classified into a read-only type, a write-once type and a rewritable type. Among them, recordable medium are the write-once type that can be recorded only once, and the rewritable type (rewrite possible type) that can be repetitively recorded and erased (can be rewritten).
A typical example of the write-once optical recording medium is CD-R having an organic dye recording layer, which penetrates widely in recent years. As the rewritable optical recording medium, a magneto-optical recording medium (magneto-optical disk) using the magneto-optical effect, and a phase-change optical recording medium (phase-change recording disk) using a difference in reflectance with a reversible change from the crystalline state to the amorphous state or vice versa are put into practice.
CD-RW, which is one of the rewritable optical recording medium, can be rewritten about a thousand times and is compatible with CD excepting the reflectance. Thus, the CD-RW widely penetrates in recent years. Medium of the rewritable type (for example, CD-RW) are more convenient for the user who prepares document files and the like as the recording medium as compared with medium of the write-once type (for example, CD-R) because the user can rewrite information sequentially.
Since an optical recording medium such as a CD-R or a CD-RW has a capacity insufficient to record motion picture data such as a movie, for example, there has been a demand for a large-capacity optical recording medium. As compared with both CD-R and CD-RW having a capacity of about 650 MB, DVD-R and DVD-RW having a capacity of about 4.7 GB, which is seven times the capacity of CD-R and CD-RW, are put into practice in recent years. As a result, use of a large-capacity DVD-R or DVD-RW enables the user to record a movie whole into one medium. Incidentally, DVD-R is a write-once optical recording medium having a dye recording layer, whereas DVD-RW is a rewritable phase-change optical recording medium.
CD-RW or DVD-RW, which is one of rewritable optical recording medium, is used differently from CD-R or DVD-R, which is one of write-once optical recording medium that can be recorded only once, according to its application. Namely, CD-RW and DVD-RW, and CD-R and DVD-R are distinguished from each other according to the application. For example, when the user needs to rewrite, the user uses a CD-RW or a DVD-R which is a rewritable optical recording medium. When there is no need to rewrite, the user uses a CD-R or a DVD-R which is a write-once optical recording medium. If the user ventures to use a medium of the rewritable type (for example, a CD-RW) as the write-once type (for example, CD-R), not only the user would fail to sufficiently use the potential of the rewritable medium, but also there is no need to use the medium of the rewritable type for it. Thus, such way of use is unusual.
As large-capacity DVD-R (hereinafter referred to as recordable DVD in this description) and DVD-RW (hereinafter referred to as rewritable DVD) are put into practical use as above, how to accomplish high-velocity recording becomes an important issue. In other words, when the capacity is increased, the size of information (data size) to be recorded is increased, which causes an increase in recording time. As compared with CD such as CD-R, CD-RW or the like having a relatively smaller capacity, DVD such as large-capacity recordable DVD, rewritable DVD or the like is required to realize high-velocity recording.
Although the recording linear velocity of the first generation recordable DVD is 3.49 m/s, a recordable DVD that can be recorded at a 4-times velocity is being put into practice. However, the 4-times velocity is considered to be still insufficient, the users thus strongly demand for a recording velocity not less than 8-times velocity.
However, the recordable DVD has a tendency that high-velocity recording causes thermal interference between recording marks. Accordingly it is said that high-velocity recording is difficult to be realized. In a recordable DVD using organic dye, a semiconductor laser beam at a wavelength in the neighborhood of 660 nm is applied to record a mark. In such recordable DVD, an increase in the recording velocity (that is, an increase in recording frequency) causes an increase in laser power required for recording as compared with the rewritable DVD, and causes a difficulty of securing a sufficient time period to allow heat generated by the laser beam for forming a recording mark to escape. For this reason, in the recordable DVD, the heat generated at the time of mark recording has a larger effect on formation of the preceding and following marks, which causes a difficulty of accurately forming recording marks. Such thermal interference causes a decrease in margin of the recording power at a recording linear velocity at a higher level in recordable DVD using organic dye. It is therefore considered that it is difficult to realize, in recordable DVD, recording at a high velocity not less than 8-times velocity.
On the other hand, the rewritable DVD does not practically have a problem because the thermal interference between recording marks is insignificant, but erasing is necessary when information is recorded (written). The rewritable recording velocity of the rewritable DVD thus still remains about 2.4 times.